Hello all
I am new here though I have been playing wesnoth for 7 months with a wesbreak.
I lost all my vets in alone at last except a necrophage and the trolls are barging in and are eliminating everything when I enter my keep .
. I am playing on easy

You should be able to reach the keep at around turn 15 or 16.. Recruit some level 1 and zombies and go to either the left or right cave and then kill some of the enemies (better yet all) there in order to level them up and in order for Malin to regain his hp. Your necrophage would be helpful as a tank in order to draw close enemy trolls then Malin makes the kill. Do not recruit bats. You should accompany them together with Malin. The left cave would have more opportunities for you to get xp to level your units, Then when you reach the center you should have some zombies or level 2 units ready to take on the troll leader. Yes I too lost all my vets in Alone at Last but the sheer power of Malin as a lich is just devastating. Do not worry about gold or finishing fast.
You can actually win this with very little gold and very few recruits/recalls, not even using up all the provided minimum gold.